1. Field of the Invention
The present invention relates to a communication system and a time synchronization method. The present invention relates particularly to a time dissemination system in a wide area or between remote locations which synchronizes a plurality of communication networks which are distributed and connected to each other through an arbitrary relay network with a single reference time server. Also, the present invention relates particularly to a communication system construction method for improving a time synchronization precision not depending on a geographical distance between the communication networks, and a signal processing method in a time control signal relay for avoiding the deterioration of a time synchronization precision associated with the indefiniteness of a communication quality between the communication networks (at the time of passing through the relay network).
2. Description of the Background Art
The internet is established as an infrastructure essential to business activities, and business data is routinely shared through the network. A sense of speed demanded for the business increases, and business operation that targets global markets is demanded more than ever before. As a result, not only backup of e-mails, web browsing, or business data, but also real-time information exchange between the remote locations through the network are increasingly required. For that reason, the enhancement of a data protection function and a further improvement in the communication quality are demanded for the network while growing data traffic is held.
In order to support real-time information sharing using voice speech or teleconference, there is a need to reduce a required data transfer time (hereinafter called “delay time”) in the case where the network is used. The diffusion of the network service based on the internet technique largely causes the data traffic to grow, and also a packet transport technique representative of an internet protocol (IP) or the Ethernet (registered in trademark) to be increasingly introduced. In order to apply a packet communication technique based on a best-effort system to a transport service provided by a communication carrier, the instability of the communication network must be reduced as much as possible. Under the circumstances, in order to improve the institution of troubleshooting or a condition monitoring, an operation, administration, and maintenance (OAM) technique and a time phase synchronization technique have been studied. In particular, in the time phase synchronization technique, its significance increases due to the rapid diffusion of mobile terminals. In order to respond to an increase in the users and an increase in the data traffic using the mobile terminals, phase synchronization among a plurality of base stations with high precision at the level of, for example, microsecond or lower is demanded. Further, time information is significant information determined in association with geographical conditions or the system specification, and associated closely with social life. A use method is conceivable in which an access time to electronic data is monitored with the use of commonality of that time to prevent falsification by a third party.
In order to use standard time information, in the related-art communication network, mainly, a standard time provision service using a network time protocol (NTP) is used, GPS receivers are installed in the respective communication networks, or time information is obtained with the use of a precision time protocol (PTP) prescribed in a standard such as IEEE 1588. The NTP represents a technique in which a standard time server (time source is a global positioning system (GPS), a standard wave, or an atomic clock) provided by an organization that manages a standard time such as National Institute of Information and Communications Technology (NICT) of Japan notifies general users of the standard time in response to a time dissemination request from client terminals such as PCs of the general users. The NTP has extensively been used in both of local area networks (LAN) and wide area networks (WAN). The precision of the NTP is about 1 millisecond, and the precision of the GPS is lower than 1 microsecond. In the NTP, the delay time (and fluctuation) of the network communication mainly causes the time synchronization precision to decrease. The IEEE 1588 PTP represents a technique in which the synchronization precision of nanosecond level is realized on condition that the PTP is mainly used in a LAN which can strictly manage the delay.
Japanese Patent No. 5122890 discloses a method for realizing the time phase synchronization with high precision between the remote locations. This publication discloses, “a transmission device creates a corrected value of the time information in each terminal device with the use of a reciprocating transmission delay time to the terminal device, and the time information of the transmission device, and transmits the corrected value. The terminal device includes information on an expected time of arrival based on the time information of the terminal device and the corrected value received from the transmission device in a frame to be transmitted toward the transmission device. The transmission device compares a receipt time of this frame with the expected arrival time information in the frame. If the receipt time is identical with the expected arrival time information, the transmission device synchronizes the time information of the terminal device with the time information of the transmission device. If not identical, the transmission device transmits a new corrected value to the terminal device. The terminal device transmits the frame including the expected arrival time information using the new corrected value to the transmission device. With the repetition of the above processing, the transmission device synchronizes the time of the terminal device with the time of the transmission device.” (refer to abstract). This is a method in which a communication device having the standard time information disseminates the time information to the plurality of terminal devices. In this example, a ranging process of a passive optical network (PON) is used for measurement of a required communication time (delay time) between the communication device and the terminal device, and a fluctuation of the delay time measured prior to the time dissemination is reduced to improve a precision of the time set in mobile units.